This invention relates to interferometry, for example, to interferometry in which the phase of light from the light source is modulated to produce interference fringes.
One known interferometer uses piezoeletric devices to move reference mirrors to provide phase modulation of the light by changing the optical path length and hence produce an interference pattern. Whilst the use of such piezoeletric devices is generally satisfactory for an interferometer working in the visible part of the electro-magnetic spectrum, such devices are not generally suitable for covering the infra-red region since the movements required of the mirrors to produce modulation are relatively large as a consequence of the longer wavelength of infra-red radiation, compared to visible radiation.
One possible way of providing for phase modulation in the infra-red region would be to utilize a plane-parallel optical flat such as is identified in U.K. Patent Specification No. 1195839, the disclosure of which is incorporated herein by reference. Although the disclosure does not relate strictly to infra-red phase interferometry, and is concerned with analysing a wide spectrum of panchromatic light, nevertheless it does teach a method whereby oscillating angular rotation of the optical flat varies the optical path length in the interferometer as a consequence of the difference in refractive index between the optical flat and the air through which the light travels inside the interferometer. However, it will be apparent that although this principle could be applied directly to a phase interferometer of known type, the change in apparent optical thickness when the angle of the flat is changed is non-linear and hence cause difficulties in interpreting the characteristics of the resulting interference pattern. This non-linearity is a consequence of the relationship relating the angle (.theta.) of the optical flat to apparent optical thickness (OT), which is given by the equation: ##EQU1## where:
T=thickness
N=refractive index of the optical flat
If this equation is plotted out it can be seen to be quite non-linear and it is for this reason that UK Patent Specification No. 1195839 also teaches the use of a correlation disc which is secured to the side of the flat for rotation about the axis of rotation of the flat. The correlation disc is provided with appropriate markings, which are spaced apart in a non-linear manner, and a photo-detector is used to read off the markings while the flat is being oscillated. The resulting signals are then processed by a computer to provide a linear reading which can then be used to derive signals corresponding to the interference pattern. The relative complexity of this approach is a disadvantage.